text: Transition metal thiolate complexes are metal complexes containing thiolate ligands.  Thiolates are ligands that can be classified as soft Lewis bases. Therefore, thiolate ligands coordinate most strongly to metals that behave as soft Lewis acids as opposed to those that behave as hard Lewis acids.  Most complexes contain other ligands in addition to thiolate, but many homoleptic complexes are known with only thiolate ligands.  The amino acid cysteine has a thiol functional group, consequently many cofactors in proteins and enzymes feature cysteinate-metal cofactors.